In recent years, because of the popularization of a small-sized portable terminal represented by a smartphone or a tablet computer, there is an increasing demand for a small-sized battery which is capable of operating these and has a higher energy density.
In general, for example, a graphite-based material is used for a negative electrode of a lithium ion battery. The theoretical capacity of a general graphite-based material is 372 mAh/g (LiC6), and the capacity of a lithium ion battery has been currently approaching this limit. Therefore, it is necessary to select a new negative electrode material which further improves the energy density of a lithium ion battery.
As a new negative electrode material, a silicon-based material and tin-based material are attracting attention. These materials have a low potential and a large specific capacity. Also, these materials can be alloyed with lithium, and thus, are preferable as a negative electrode material. Among these materials, a silicon particle can insert lithium up to 4.4 lithium atoms with respect to 1 silicon atom in a ratio, and can theoretically have 10 times the capacity of a graphite-based carbon material.
However, when a silicon particle inserts lithium atoms, the volume expands by about 3 to 4 times. For this reason, in the battery obtained by using a silicon particle as a negative electrode material, the repetition of charge and discharge cracks and pulverizes a silicon particle, and affects the other members constituting the electrodes, and thus, sufficient cycle characteristics may not be obtained. In terms of suppressing the pulverization of a silicon particle, it is effective to reduce the particle size of a silicon particle to 150 nm or less and to decrease the effect based on the volume expansion. However, fine silicon particles are easily aggregated to each other. Therefore, the measures such as the coating of the periphery of a silicon particle with a carbon layer to prevent the aggregation have been proposed.
Also, in the battery obtained by using a silicon-based active material as a negative electrode, the first-time charge and discharge efficiency may not be sufficiently obtained. This is because the 100% lithium, which has been inserted in a silicon-based active material of a negative electrode at the first-time charge, is not returned to a positive electrode at discharge, which is a so-called irreversible capacity. The presence of an irreversible capacity of a negative electrode may result in the decrease in energy density when a battery is produced by combining a negative electrode and a positive electrode. This is because the lithium for an irreversible capacity of a negative electrode should be preliminarily prepared in a positive electrode in order to achieve a battery having high capacity, which results in the decrease in energy density. Therefore, in the battery obtained by using a silicon-based active material as a negative electrode, the improvement of the first-time charge and discharge efficiency is indispensable in order to improve the energy density.